For measuring a height difference between two points at a high accuracy, measurement has been performed by mounting a round bubble vial on a leveling rod, an operator holding the leveling rod vertically while observing the round bubble vial, and another operator reading the graduations on the leveling rod by a level. On the other hand, at the time of survey operation, not only has it been necessary to mount the round bubble vial on the leveling rod every time for measurement, but there has also been a troublesome operation, such that accuracy confirmation of the round bubble vial using a plumb bob or the like is also required. Therefore, as shown in FIG. 1, a measuring method called a waving method in which an operator finds and determines as a measured value the minimum reading of the graduations on the leveling rod 2 while rocking the leveling rod 2 back and forth toward the level 1 without mounting a round bubble vial on the leveling rod 2 has also been commonly performed.
Meanwhile, in recent years, electronic levels that allow automatic reading of the graduations on the leveling rods have also appeared. The electronic level is a level structured so as to allow automatic reading of the graduations by using a leveling rod with a barcode as the graduations. However, at present, no electronic levels compatible with the waving method are available, which has been one of the obstacles hindering the spread of electronic levels.
To respond to such a situation, the following patent literature 1 discloses an electronic level compatible with the waving method.
The electronic level disclosed in the following patent literature 1 will be described based on FIG. 5. With this electronic level, the shoe of a leveling rod 2 is fixed to a measuring point, an image S of the leveling rod 2 is projected onto a two-dimensional sensor 17 while the leveling rod 2 is rocked back and forth toward the electronic level. Then, by scanning the image S along a scanning line CT parallel to the Y-axis, a signal waveform Z according to a graduation pattern drawn on the leveling rod 2 is obtained. The signal waveforms Z obtained at a predetermined sampling cycle have changes in the signal waveform Z as shown in FIG. 5 (a), (b), and (c). The position on the scanning line CT of an identifying point P on the leveling rod 2 is determined from these signal waveforms Z, and when the identification point P has moved most to one side (side corresponding to a higher position of the leveling rod 2) on the scanning line CT, a graduation pattern on the leveling rod 2 captured by a two-dimensional sensor 17 is stored in an image memory (not shown). Here, by reading images of the graduation patterns stored in the image memory, a height difference between two points can be measured.